Olefins such as ethylene and propylene are extremely valuable industrial products both as monomers for homo- and co-polymer production and as a basic starting material for the production of other desirable chemicals. Such olefins may be obtained through various processes which include the steam cracking of hydrocarbons such as naphtha and liquid petroleum gas feedstocks or through the dehydrogenation of paraffinic hydrocarbons.
European patent application 0332289 discloses a method for the production of mono-olefins which involves the partial oxidation of paraffinic hydrocarbons. Gaseous paraffinic hydrocarbons are mixed with a molecular oxygen-containing gas and contacted with a catalyst capable of supporting combustion beyond the fuel rich limit of flammability. The process, hereinafter described as autothermal cracking provides an efficient and cost-effective method of producing mono-olefins with yields being in excess of that obtained through the conventional steam cracking technology.
The aforementioned process is successful for gaseous hydrocarbon feeds such as ethane, propane or butane. The process is equally applicable to liquid hydrocarbons such as naphtha, gas oil or vacuum gas oil. The liquid hydrocarbons, however, are commonly preheated and partially vaporised prior to entering the reaction chamber, thus requiring an additional step in the process. A problem associated with vaporisation of liquid hydrocarbons is that the conditions, e.g. high temperature, required to achieve vaporisation of heavier feeds may cause cracking of the hydrocarbons. Additionally, complete vaporisation is difficult to achieve and consequently a fine film of liquid can form on the catalyst which can reduce catalyst activity.
Attempts to overcome the aforementioned problem fall into two general categories. Firstly, additional oxygen may be used to promote the combustion and to raise the temperature of the catalyst. This, however, results in a decrease in the yield of olefins. Alternatively, greater heat may be applied. In addition to the problem of cracking of the hydrocarbons, additional pre-heat may result in flashback or autoignition generating a flame at the fuel/oxygen mixing point. This type of flame generates very high flame temperatures and may result in carbon formation and thus a decrease in the yield of olefins.